Morphine-6-glucuronide synthesis

ABSTRACT

The invention provides a novel method for synthesizing Morphine-6-Glucuronide comprising the step of reacting 3-O-pivaloyloxymorphine and methyl 1α,2-ethylorthopivalate-3,4-di-O-pivaloylglucouronate.

This is a divisional of Application Ser. No. 09/700,909, filed Mar. 9,2001 which is a 371 of PCT/GB99/01777, filed Jun. 4, 1999, all of whichare incorporated herein by reference.

The invention provides a novel method for synthesisingMorphine-6-Glucuronide (M6G) and intermediates therefor.

Synthesis of M6G from 3-acetyl morphine and methyl2-α-bromo-3,4,5-tri-O-acetylglucuronate is described by Lacy, C., et al.(Tetrahedron Letters, 36 (22), (1995), 3949-3950).

Hidetoshi, Y. et al., (Chemical and Pharmaceutical Bulletin, JP, TOKYO,16 (11), (1968), 2114-2119) describe synthesis of M6G by reaction of3-acetyl-morphine with a bromo derivative of glucuronic acid to form aMethyl [3-acetyl-morphine-6-yl -2,3,4-tri-O-acetyl-β-D-glucopyranosid]uronate intermediate which is subsequently hydrolysed to M6G.

WO 93/05057 discloses preparation of M6G by reaction of 3-acetylmorphine with methyl 1α-bromo, 1-deoxy, 2,3,4-tri-O-acetyl Dglucopyranuronate and subsequently hydrolysing the resultingintermediate to M6G.

In order to synthesise M6G the major problem to overcome is to obtainthe glycoside linkage with very high β-selectivity since prior methodsproduce the α-anomer.

One method for obtaining high β-selectivity is to use trichloroimidateas the leaving group, as shown in WO 93/03051: FIG. 1 (Salford UltrafineChemicals and Research Limited).

Orthoesters are simple to synthesise from their respective bromides.There is a reaction reported in the literature² between the glucuronateorthoester (2) and the sugar derivative (3) catalysed by lutidiniumperchlorate³ (4) (Scheme 1).

When this reaction was repeated with the t-butyl orthoacetate (5) andcyclohexanol (6 equivalents), the desired product (6) was isolated in 9%yield. Two other products also suggested that they were the desiredproduct, but with the loss of one acetyl group, isolated in a combinedyield of 43% (Scheme 2).

When 1.2 equivalents of 4-tert-butylcyclohexanol was used, the desiredcompound (7) was obtained in 17% yield. Other compounds obtained fromthe reaction also appeared to contain the desired peaks in the nmr, butafter further examination proved to be the product oftransorthoesterification (8) (Scheme 3).

Reaction of Orthoester (5) with Protected Morphine

Initially, 1.2 equivalents of 3-TBS protected morphine and theorthoester (5) were dissolved in chlorobenzene and half of the solventwas distilled off before 0.1 equivalents of lutidinium perchlorate (4)in chlorobenzene was added. The solvent was continuously distilled offwhile fresh solvent was added, and after 2.5 h another compound wasformed with similar tlc properties to the protected morphine. Workup andchromatography gave a compound which corresponded totrans-orthoesterified material (9). None of the desired material wasobtained (Scheme 4).

This product (9) was resubmitted to the reaction conditions (0.1equivalents of lutidinium perchlorate and protected morphine inrefluxing chlorobenzene) with no new products formed after 4 h. Twofurther reactions were attempted using two equivalents of orthoester (5)and 0.2 equivalents of lutidinium perchlorate and 1 equivalent oforthoester (5) and 1.2 equivalents of lutidinium perchlorate, but bothgave varying yields of orthoester (9).

We have concluded that a different, more bulky, alkyl group was neededon the orthoester to hinder attack there. Initially, the isopropyl groupwas examined. However, the initial reaction, perisobutyrylation, failedto give a compound which recrystallised from petrol, so the α and βanomers could not be separated. Therefore, attention focussed on thepivaloyl group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art process.

FIG. 2 is a reaction scheme according to the invention for synthesizingM6G.

The invention is further described with reference to the accompanyingFIG. 2 which shows a summary of a reaction scheme according to theinvention for synthesising M6G.

Synthesis of the Perpivalated Glucuronide

Synthesis of perpivalated glucuronide proved troublesome at first,giving a mixture of 3 and 4 non-pivalated material (scheme 5).

A search through the literature revealed that glucose can beperpivalated by heating the reaction to reflux for 3 h. and thenstirring it for 7 days.

When this reaction was repeated on ring-opened glucurono-3,6-lactone(Scheme 6), perpivalated product (10) was obtained by crystallisation ofthe crude product from MeOH (or EtOH) and water and drying the crystalsby dissolving them in DCM, separating any water present, drying, andthen evaporating the organic layer to give the product in 29-52% yield,a substantial improvement on previous yields for this step.

DMAP was added to aid perpivalation, although there has been no evidenceto suggest that this is necessary. The variation in the yields quoted isprobably due to the amount of MeOH left over from the first step. Thehigh yield quoted (52%) was obtained by using 6 (instead of 5)equivalents of tBuCOCl. A slight colouration of the final product provedno handicap in the next step, as after a silica plug andrecrystallisation, pure white crystals were obtained.

Synthesis of the Orthoester (6)

Conversion of the perpivalated material (10) to the α-bromide (11)required gentle heating (to approxiamately 35° C.) to dissolve thesubstrate in the reaction mixture. The reaction proceeded very cleanlyby tic analysis, showing a spot to spot conversion. Attempts to reducethe amount of HBr used to five equivalents led to incomplete conversionof the starting material, so 12 equivalents were used as before. Theproduct was slowly crystallised from EtOH/water or MeOH/water to givelong white crystals in a yield of 52-78%. High yields were alwaysobtained when fresh HBr/AcOH was used. The crystals were dried by againdissolving them in dichloromethane, the water separated, and the organiclayer dried and evaporated.

The orthoester (12) was obtained in 63-81% yield by stirring a 1:1mixture of EtOH:collidine at 70° C. (oil bath temperature) with thebromide (11) and 0.8 equivalents of Et₄NBr (Scheme 7). The product caneasily be crystallised from EtOH/water water or MeOH/water as whitecrystals, with a trace of collidine still present (detected by smell!)but which doesn't effect the next reaction. An interesting by-productfrom this reaction (obtained in about 10%) is the result of EtOHattacking the anomeric position to give the β-anomer (13) Again, thedifficulty in drying the crystals meant that they were dissolved inpetrol (40-60), the water separated, and the organic layer dried andevaporated.

Synthesis of 3-Pivalated Morphine (14)

Selective deprotonation of the phenolic OH of morphine was achievedusing NaH (surprisingly, the anion turns out to be soluble in THF) andtrimethylacetyl chloride was added dropwise to give the desired productafter recrystallisation from MeOH/water (Scheme 8). Again, thedifficulty in drying the crystals meant that they were dissolved indichloromethane, the water separated, and the organic layer dried andevaporated to give a white powder in 81% yield.

1.1 equivalents of trimethylacetyl chloride were used, but this led tosome dipivalated morphine which proved difficult, to recrystallise apartfrom mono-pivalated morphine (14) or the protected M6G (16). Thus, itwould be advantagous in the future to use 1 equivalent oftrimethylacetyl chloride.

Synthesis of Lutidinium Perchlorate (15)

This was achieved by simply adding aqueous perchloric acid to an ethersolution of lutidine (excess, as this remains in the Et₂O layer) (Scheme9) and evaporating the water until crystals form, which were collectedby filtration.

The crystals are deliquescent and thus need to be dried under highvacuum prior to use.

Other acid catalysts have been investigated in the coupling reactionbelow, but with no success. However, this compound has shown notendencies to decompose, proving both thermal and shock stable, soshouldn't prove a problem on scale up.

Coupling of the Orthoester (12) with 3-pivalated Morphine (14)

Coupling the orthoester (12) to 1.1 equivalents of 3-pivalated morphine(14) was achieved by adding 0.1 equivalents of lutidinium perchlorate(15) every 15 min. until 1.2 equivalents had been added to thedistilling chlorobenzene. The reaction was then stirred under reflux fora further 2 h to give a mixture of 3-pivalated morphine (14) protectedM6G (16) and much less polar materials. Work-up and crude purificationby chromatography gave protected M6G (16) and 3-pivalated morphine (14)which was purified by recrystallisation from MeOH/(water, smallquantity) to give (16) in 29% yield (with no detectable quantity ofα-anomer or trans-orthoesterified material from nmr analysis) (Scheme10).

This yield is the greatest amount obtained from this reaction andfurther improvements might be possible. Lutidinium perchlorate (15) wasadded every 15 min. as a solid appeared to crystallise from the reactionmixture (presumably the 3-pivalated morphine perchlorate) and, if nomore catalyst is added, the major product turned out to be thetrans-orthoesterified material (similar to orthoester (9) in Scheme 14).If 1.2 equivalents of lutidinium perchlorate (15)was added directly tothe reaction, only 6% of coupled material was obtained (presumably asall the 3-pivalated morphine had been removed from the reaction as theperchlorate salt). The main problem with adding the catalyst, is itsinsolubility in chlorobenzene lower than approxiamately 100° C. If it ispossible on a large scale to add lutidinium perchlorate (15) inchlorobenzene at 100° C., this may prove not only simpler to add thecatalyst, but also lead to increasing yields. The reaction also needs tobe refluxed for an additional 2 hours after all the lutidiniumperchlorate (15) has been added, to cause the trans-orthoesterifiedmaterial to rearrange to the desired material.

Global Deprotection of Protected M6G (16)

Heating protected M6G (16) in MeOH until it dissolves before adding thewater (which causes it to crystallise from the reaction mixture) andCa(OH)₂ seems to be the mildest way of performing this reaction. Afterstirring for 3 days, the reaction gave, by tic analysis, M6G (17) andmorphine (Scheme 11).

The reaction was quite slow due to the insolubility of Ca(OH)₂ in water,but when the reaction is deemed to have finished by tic analysis, 6.5equivalents of sulfuric acid were added or until the reaction reached pH4. The CaSO₄ so formed was filtered off and the trimethylacetic acidalso formed was removed by washing the filtrate with DCM. Evaporatingthe water proved the hardest part of this reaction due to excessivefoaming. Some CaSO₄ remains in the filtrate and this was removed byadding MeOH to crystallise it out. The residue produced after all thewater had been evaporated was purified by repeated washing with MeOH asM6G is virtually insoluble in MeOH while morphine is soluble in it. Themorphine present in the crude residue probably arrived there due to thedi-pivalated morphine passing through the coupling reaction and thenbeing deprotected to morphine in this final step. Hopefully, by usingstrictly 1 equivalent or trimethylacetyl chloride, this should eliminatethe di-pivalated morphine, thus make purification of M6G even simpler,and increasing the yield for the final step.

The invention is further described in detail below by way of exampleonly.

EXAMPLE 1

Methyl 1β,2,3,4-tetra-O-pivaloylglucuronate

Glucurono-6,3-lactone (147 g, 0.8 mol) was stirred as a suspension inmethanol (1 L, not dried) under nitrogen. A catalytic amount of sodiummethoxide (147 mg, 2.6 mmol) was added to the suspension, and after 2hours most of the suspension was still present. The reaction proceededvery slowly at room temperature, ˜18° C., but noticeably increased inrate when the reaction was warmed, therefore, the reaction was gentlywarmed to ˜25° C. After another hour of stirring, most of the suspensionhad dissolved to leave a clear yellow solution that was then evaporated.The residue was found to be a solid, which tended to foam under vacuum,which made total removal of all the methanol difficult.

Chloroform (400 mL), followed by 6 equivalents of pyridine (400 mL, 4.8mol) and a catalytic amount of N,N-dimethyl-4-aminopyridine (4 g) wasthen added to the residue that slowly dissolved in this mixture. Thesolution was stirred using a magnetic stirrer plate and flea, but theproblems encountered in continuously stirring this reaction would makean overhead mechanical stirrer preferable at this stage. The reactionwas then cooled to 0° C. and 5 equivalents of trimethylacetyl chloride(500 mL, 4 mol) was added gradually, not allowing the reaction to warmto a temperature above ˜8° C. The yellow/orange solution becamecolourless on addition of the first portion of trimethylacetyl chloride,and alter approximately half of the volume was added, a whiteprecipitate was observed (pyr.HCl). After addition was complete, thereaction was stirred overnight at room temperature before being heatedat reflux for 2 hours, during which time the reaction turned black withthe white precipitate still present. Tlc analysis showed that thedesired product had been produced (Rf 0.5, 1:1 Et₂O:petrol), but somemono-unprotected material remained (Rf 0.3 and 2.8, 1.1 Et₂O:petrol).The reaction was then allowed to cool to room temperature over 3 hours,then further cooled to 0° C. before methanol was added gradually (thisquenches the excess trimethylacetyl chloride to give methyltrimethylacetate, which is evaporated off with the solvent). The blacksolution was then poured into a 2 L separating funnel, and washed withwater (600 mL), 1M HCl (2*600 mL), water (600 mL), and saturated aqueousNaHCO₃ (2*600 mL). The organic layer was then dried with MgSO₄ andpassed through approximately 5 cm of silica on a sinter funnel (whichremoved a black baseline compound). The silica was washed withdichloromethane (100 mL) and the combined filtrates evaporated to leavea black viscous oil, which was re-dissolved in ethanol (˜1 L) and hadwater added until the solution turned turbid (˜500 mL). More ethanol wasadded until the turbid solution cleared, and the solution was left tocrystallise overnight. The yellow crystals were dissolved indichloromethane (300 mL) and any excess water removed by separation, thedichloromethane layer Was then dried and evaporated.

The white powder (113.5 g, 26%) was then used in the next reaction.

Methyl 1-deoxy-1-α-bromo,2,3,4-tri-O-pivaloylglucuronate

Methyl 1(β),2,3,4-tetra-O-pivaloylglucuronate (108.5 g, 0.2 mol) wasdissolved in glacial acetic acid (500 mL) (with the aid of some gentleheating) and placed in a bath of cold water. 12 equivalents of 33% HBrin acetic acid (500 mL, 2.9 mol) were then added at a rate required toprevent the acetic acid freezing without the reaction exotherming toogreatly. After the addition was complete, the reaction was allowed towarm to room temperature. If any white solid (starting material)persisted, gentle warming was applied to the reaction until it dissolvedand the reaction then allowed to cool and stir overnight. Theorange/brown solution was then cautiously poured into dichloromethane(500 mL)/water (500 mL), the organic layer separated, washed with water(500 mL) and saturated NaHCO₃ (500 mL) (with care to avoid too rapid anevolution of CO₂). The organic layer was then dried (MgSO₄) and passedthrough approximately 2 cm of silica, the silica was washed with moredichloromethane (50 mL) and the combined filtrates evaporated (takingcare to remove all the dichloromethane). The residue was then dissolvedin EtOH (˜400 mL) and water added until the reaction turned turbid. Moreethanol was added until the solution just turned clear and the productallowed to crystallise overnight which were collected by filtration. Thecrystals were dissolved in dichloromethane and the organic layerseparated from any water that remained, dried (MgSO₄), and evaporated.

The white powder (76 g, 72%) was then used in the next reaction.

Methyl 1α,2-ethylordhopivalate-3,4-di-O-pivaloylglucuronate

Methyl 1-deoxy-1-α-bromo,2,3,4-tri-O-pivaloylglucuronate (69 g, 0.13mol) was dissolved in collidine (300 mL) (pre-dried by distilling ontoactivated 3 Å sieves) and ethanol (300 mL) (pre-dried by distilling fromNaOEt onto activated 3 Å sieves). 0.8 equivalents of pre-driedtetraethylammonium bromide (22 g, 0.1 mol) was then added to thereaction, which was stirred at 60° C. (oil bath temperature 70° C.)overnight. The reaction was then cooled and poured into dichloromethane(500 mL)/water (500 mL) and the organic layer separated, dried (MgSO₄),and evaporated. The collidine was removed by low-pressure distillation(total evaporation is not necessary), the residue dissolved in EtOH(˜400 mL), and water added until the product started to crystallise out.The white crystals were collected by filtration and dissolved in petrol.The organic layer was then separated from any water that remained, dried(MgSO₄), and evaporated.

The white powder (50 g, 78%) was then used in the next reaction

3′-O-Pivaloylmorphine

Morphine (12 g, 42 mmol) was added portionwise to a THF (80 mL, Nadried) suspension of 1.05 equivalents of petrol washed NaH (60%dispersion in oil, 1.768 g, 44 mmol) at 0° C. After stirring for 1 h atroom temperature, 1.1 equivalents of trimethylacetyl chloride (5.7 mL,46 mmol) were added to the clear reaction mixture at 0° C. andeventually a white solid precipitated from the reaction. After 1 h⁻.,MeOH⁻ (10 mL) followed by saturated aqueous sodium bicarbonate (100 mL)were added to the reaction which was then extracted with Et₂O (2×200mL). The combined extracts were washed with brine (200 mL), dried, andevaporated. The residue was recrystallised from MeOH/water and thecrystals dissolved in dichloromethane and the organic layer separatedfrom any water that remained, dried (MgSO₄), and evaporated.

The white powder (12.6 g, 81%) was then used in the next reaction.

Lutidinium Perchlorate

A 60% aqueous solution of perchloric acid (29 mL, 0.27 mol) was added to1.1 equivalents of lutidine (34 mL, 0.29 mL) in Et₂O (250 mL) at 0° C.After stirring for 0.5 h. at room temperature, the aqueous layer wasseparated and the water evaporated until a white solid crystallised fromthe water, the crystals filtered off and washed with Et₂O to give theproduct as a white crystalline solid (30 g, 54%). The product was driedunder high vacuum prior to use.

Methyl 1β-6′-O-(3′-O-pivaloylmorphine)-2,3,4-tri-O-pivaloylglucuronate

A chlorobenzene (400 mL) (distilled from CaH₂ onto activated 3 Å sieves)solution of 1.1 equivalents of 3-O-pivaloyoloxymorphine (8.49 g, 23mmol) and methyl 1α,2-ethylorthopivalate-3,4di-O-pivaloylglucuronate (10g, 20 mmol) was heated to reflux to distil off approximately half of thesolvent. 0.1 Equivalents of lutidinium perchlorate (415 mg, 2 mmol) wasthen added to the reaction that was still at reflux. The reaction wasthen stirred at reflux for 15 min with chlorobenzene continuouslydistilled off and fresh chlorobenzene added. After this time, a further0.1 equivalents of lutidinium perchlorate (415 mg, 2 mmol) was thenadded to the reaction. This procedure was repeated every 15 min until1.2 equivalents of lutidinium perchlorate (5.2 g, 25 mmol) had beenadded. The reaction was then stirred at reflux for 2 hours withchlorobenzene continuously distilled off and fresh chlorobenzene added.After this time, the reaction was allowed to cool and then poured intodichloromethane (500 mL)/water (500 mL), the organic layer separated,washed with saturated aqueous sodium bicarbonate (500 mL), dried, andevaporated. The residue, after some of the chlorobenzene had beenremoved under low pressure, was applied to the top of a silica columnand eluted with diethyl ether to remove the non-polar by-products andthen with 5% methanol in dichloromethane. The desired product wasseparated from 3-Piv-M by recrystallisation from MeOH/water to give awhite crystalline powder (4.76 g, 29%).

Morphine-6-gluconoride

Methyl1β-6′-O-(3′-O-pivalayloxymorphine)-2,3,4-tri-0-pivaloylglucuronate (3.06g, 3.77 mmol) was dissolved in MeOH (60 mL) (with the help of someheating) and had water (7 mL) followed by 6.5 equivalents of calciumhydroxide (1.817 g, 24.5 mmol) added to it. The reaction was stirred fortwo days when water (60 mL) was added and the reaction stirred for afurther day until the reaction was shown to be complete by tic analysis(Rf 0.3, 45% nBuOH; 15% water; 20% acetone; 10% acetic acid; 10% of a 5%aqueous solution of ammonia). 6.5 equivalents of 0.25M aqueous sulphuricacid (98 mL, 24.5 mmol) were added (pH 4) and the reaction stirred for 1hour. The reaction was then filtered to remove CaSO₄ and the solidwashed with water (30 mL). The filtrate was then washed with DCM (2×100mL), three quarters of the water evaporated and the same quantity ofMeOH added. The white solid (mainly CaSO₄) was then filtered and thefiltrate evaporated. The residue (1.56 g) had MeOH (100 mL) added andthe white solid filtered and repeatedly washed with MeOH to give thedesired compound (1.05 g, 60%) which could, according to the literature,be recrystallised from H₂O/MeOH (although this has not been performed onthis material).

References

1. For a review of orthoesters and their synthetic applications see N.K. Kochetkav and A. F. Bochkov, Recent Developments in the Chemistry ofNatural Carbon Compounds, Ed. R. Bognár, V. Bruckner, and Cs. Szántay,Akadémiai Kiadó: Budapest, 1971, vol. 4, p.77 191.

2. H. P. Wessel, L. Labler, and T. S. Tschopp, Helv. Chim. Acta., 1989,72, 1268.

3. The use of 2,6-dimethylpyridinium perchlorate (4) was first reportedby N. K. Kochetkov, A. F. Bochkov, T. A. Sokolovskaya, and V. J.Snyatkova, Carbohydr. Res., 1971, 16, 17.

What is claimed is:
 1. A compound or derivative thereof of the followingformula: